help importing custom textures

well, I think that at one point I knew how to import custom textures, and to make them usable in the 3dmode of doom builder, but, if I still knew, I wouldn't be asking :P
Anyway, I'm mapping for the STLMSv3 wad, to be released on X-mas
and I'm making a CPU arena. I rly need to know how to add textures. I remember it had something to do with margins :P thats all X_X
thnx for any help!